The 7805 blows at this low V_in thing and the LDO seems to hold up,but I'm not convinced that there isn't a "gotcha" in there.Anyway, such are my findings.

"You gotta fight -- for your right -- to party!"Don't react - Read."Who is like unto the beast? who is able to make war with him?"Hey, it's "bipolar transistor" or "junction transistor" - "BJT" is just stupid.

Well, the OP said his max expectation was for "100 mA",but I may take a look at that.The 2940 output with 47? showed some differencefrom 60?, I checked and re-checked it.

"You gotta fight -- for your right -- to party!"Don't react - Read."Who is like unto the beast? who is able to make war with him?"Hey, it's "bipolar transistor" or "junction transistor" - "BJT" is just stupid.

Runaway Pancake, thanks for your chart there. It's nice to back up with experimentation what everyone was saying would happen.

Quote from: Jack Christensen

Late to this party, questions for the OP. How long to the AA cells need to last? What is the source of the 12V supply?

I'm planning a timelapse controller. It probably would only be run for 4 hours maximum at a time. The 12V would be from a SLA or Li-ion battery or whatever the user wants to take into the field for power.

A buck-boost regulator comes to mind. Haven't actually done one myself, but I've had good results with a small boost converter (MCP1640) in a circuit that requires right around 100mA at 5V. It can use two or three alkaline or NiMH AA cells. Haven't done a real good test yet, but it looks like two fresh alkaline cells should run it for at least 8 hours, three should give half again that. Makes a lot of sense to boost a lower voltage when it can be done at 90%+ efficiency, as opposed to knocking it down with a linear regulator and wasting power. A lot of the switching regulators these days don't require complex circuitry. Build it on a PC board, keep things close and traces short and you're off to the races.

I definitely would try to avoid the scenario of running a regulator (or most anything) out of spec.

That's a fantastic solution. I will definitely look into it. Based on the datasheet, the additional circuitry for the MCP1640 is minimal, and they even have a recommended PCB layout.

The only problem is this eliminates the external 12V option, but I can still have a linear regulator for that. Or would anyone have an example of a switching regulator with a 2-12V input and 5V output?

Edit: Anyone have any comments about the MIC2570?http://www.digikey.ca/product-detail/en/MIC2570-1YM/576-2190-ND/1028727

Took a quick look at the MIC2570 datasheet, it's a boost regulator as is the MCP1640, I'm not 100% sure, but my initial assumption would be that the input voltage cannot exceed the output voltage (same as MCP1640). Other than that, it looks fine. TI makes some interesting buck-boost regulators, they're in very small surface mount packages though, if that's an issue.

Using a linear regulator on the 12V input also occurred to me. Feels a bit kludgey, but if the low efficiency is not an issue (and it might not be for a SLA, etc.) then that could work.

I have a small PC board which I use as a breadboard power supply that uses the MCP1640. Happy to share, if you're interested, PM me.

I am a bit lost finding a suitable inductor. I don't understand saturation current or ESR. Could you link a suitable inductor that digikey supplies? I would like an SMD part, but I will be soldering by hand, so the leads can't be on the bottom.

For the capacitors would these work? http://www.digikey.ca/product-detail/en/C2012Y5V1A106Z/445-1371-1-ND/567608